Textbox with an unknown value

I have a textbox where the user can enter a number. In the past, an empty value would indicate that they didn't know the value. Now, the client wants to add an extra step where the user needs to specify that they don't know the value, otherwise the textbox should be marked as mandatory. They think this would increase the chance of people filling in the textbox.

I'm trying to limit space to make sure the form isn't too large. I'm looking for a bunch of different ideas. I thought of adding a checkbox beside the textbox (I'm using Bootstrap), which would set the textbox as disabled.

But it doesn't look intuitive. I don't want a bunch of people calling the helpdesk.